Toxic sludge that burst from a containment reservoir in western Hungary has reached a branch of the Danube, and experts are trying to assess the environmental impact on Europe's second largest river.

Authorities say the sludge reached the Mosoni-Danube early Thursday. A Hungarian disaster management official, Tibor Dobson, said water samples show alkalinity levels above normal, but that those are dropping as river waters dilute the caustic slurry.

European Union and environmental officials are worried that the spill could wreak havoc on the Danube's ecosystem and six downstream countries. On Thursday, EU environmental spokesman Joe Hennon in Brussels said the threat has not yet been fully assessed.

Dobson said all fish and wildlife in the first-hit Marcal river are dead. Emergency crews are working to neutralize the sludge.

Hungarian Prime Minister Viktor Orban has described the spill as a "serious ecological disaster." On Thursday Mr. Orban visited three villages destroyed by the deluge and he declared the area a total loss.

Hungary on Wednesday opened a criminal investigation into the spill, caused when a containment reservoir at a nearby metals factory burst. Four people were killed and more than 100 others injured, as a torrent of toxic red mud roared through several nearby villages.

Authorities said clean-up could take a year.

The red sludge is a byproduct of processing bauxite, a basic material for manufacturing aluminum. It is laden with caustic soda, heavy metals and other hazardous chemicals that can burn people through their clothes, and corrode metal and other materials. If the toxic sludge dries out, it could turn into dust and spread through the air.

Police are investigating to determine why a reservoir wall broke, and whether negligence contributed to the disaster.